Gotowa bibliografia na temat „Co3(Al,W)”

Site occupancy of Y in the γ′-Co3(Al,W) was predicted theoretically by first-principles calculations based on density functional theory. By computing total energy as a function of applied strain, the elastic constants of quaternary Co3(Al,W) were also predicted. The results suggest that Y preferentially occupies the W sites in Co3(Al,W). The calculation of heat of formation shows that the occupancy of Y on the W sites decreases the phase stability of Co3(Al,W). The theoretical calculation also shows that the L12 Co24Al4W3Y compound is ductile in nature.

The physical and mechanical properties of Co3(Al,W) with the L12 structure have been investigated both in single and polycrystalline forms. The values of all the three independent single-crystal elastic constants and polycrystalline elastic constants of Co3(Al,W) experimentally determined by resonance ultrasound spectroscopy at liquid helium temperature are 15~25% larger than those of Ni3(Al,Ta) but are considerably smaller than those previously calculated. When judged from the values of Poisson’s ratio, Cauchy pressure and Gh (shear modulus)/Bh (bulk modulus), the ductility of Co3(Al,W) is expected to be sufficiently high. Indeed, the value of tensile elongation obtained in air is as large as 28 %, which is far larger than that obtained in Ni3Al polycrystals under similar conditions.

First-principles calculations have been performed to investigate the phase stability, elastic, and thermodynamic properties of Co3(Al,Mo,Ta) with the L12 structure. Calculated elastic constants showed that Co3(Al,Mo,Ta) is mechanically stable and possesses intrinsic ductility. Young’s and shear moduli of polycrystalline Co3(Al,Mo,Ta) were calculated using the Voigt-Reuss-Hill approach. It was found that the shear and Young’s moduli of Co3(Al,Mo,Ta) were smaller than those of Co3(Al,W). States density indicated the existence of covalent-like bonding in Co3(Al,Mo,Ta). Temperature-dependent thermodynamic properties of Co3(Al,Mo,Ta) could be described satisfactorily using the Debye-Grüneisen approach, including entropy, enthalpy, heat capacity and linear thermal expansion coefficient, showing their significant temperature dependences. Furthermore the obtained data could be employed in the modeling of thermodynamic and mechanical properties of Co-based alloys to enable the design of high temperature alloys.

The mechanical properties of Co3(Al,W) with the L12 structure have been investigated both in single and polycrystalline forms. The values of all the three independent single-crystal elastic constants and polycrystalline elastic constants of Co3(Al,W) experimentally determined by resonance ultrasound spectroscopy at liquid helium temperature are 15~25% larger than those of Ni3(Al,Ta) but are considerably smaller than those previously calculated. When judged from the values of Poisson’s ratio, Cauchy pressure and ratio of shear modulus to bulk modulus (Gh/Bh), the ductility of Co3(Al,W) is expected to be sufficiently high. In the yield stress-temperature curve, a rapid decrease and an anomalous increase in yield stress is observed in the low and intermediate (1000-1100 K) temperature ranges, respectively. The former is concluded to be due to the solid-solution hardening effect while the latter is attributed to thermally activated cross-slip of APB-coupled a/2<110> superpartial dislocations from octahedral to cube slip planes.

The results of the experimental study of the mechanical properties and structure of the Co-9.5Al-2.9Mo-4Nb, Co-9.1Al-5.2Mo-4.7Nb, and Co-8.9Al-6.5Mo-9.3Nb alloys were presented. The Young’s moduli in the studied alloy samples were found to be smaller than those of Ni3Al-based and Co3(Al,W)-based alloys. The eutectic structure was observed in all studied alloys. Cuboids of the Co3(Al,Nb,Mo) intermetallic compound with L12 crystal structure were found by TEM study.

ABSTRACTIt is well known that various elements substitute for a certain sub-lattice of intermetallic compounds. There have been various experimental investigations of the effects of substituted elements on mechanical properties, however, there are few reports describing the effects of multi-element substitution. In the present study, L12-type compounds A3B (Ni3Al and Co3(Al,W)) were selected as model compounds because their substitution behavior is well known. It was reported that various elements such as Ni, Co, Cu, Pd and Pt occupy the A-site, whereas Al, Si, Ga, Ge, Ti, V, Nb, Ta, Mo, and W occupy the B-site. These elements are expected to introduce local lattice distortion, which may affect the motion of dislocations over a wide range of temperatures. Several alloys composed of five or more elements including Ni, Co, Al, Mo, and W, were prepared using an Ar-arc melting machine and heat-treated. Several alloys were found to include an (Ni, Co)3(Al, Mo, W, …)-L12 compound as a constituent phase. The nano-hardness of these L12 phases was higher than that of the high-strength Co3(Al,W)-L12 compound, confirming that multi-element substitution is an effective way to improve the mechanical properties of an intermetallic compound without decreasing the phase stability.

The values of all the three independent single-crystal elastic constants and polycrystalline elastic constants of Co3(Al,W) experimentally determined by resonance ultrasound spectroscopy at liquid helium temperature are 15~25% larger than those of Ni3(Al,Ta) but are considerably smaller than those previously calculated. Because of the large value of E111/E100 and cij of Co3(Al,W), two-phase microstructures with cuboidal L12 precipitates well aligned parallel to <100> and well faceted parallel to {100} are expected to form very easily in Co-base alloys, as confirmed indeed by experiment. Values of yield stress obtained for [001]-oriented L12/fcc two-phase single crystals moderately decrease with the increase in temperature up to 800°C and then decrease rapidly with temperature above 800°C without any anomaly in yield stress.

High temperature metallic structural materials, such as Ni-base superalloys owe their strength to a two-phase microstructure consisting of an fcc matrix strengthened by intermetallic A3B type precipitates. The performance of these alloys derives from the exceptional high temperature strength of the L12-ordered A3B precipitate, which in turn is strongly influenced by planar faults created when the precipitate is sheared. In this context, the broad goal of this work is to understand the role of composition on planar fault energies in A3B compounds with L12 structure. Towards this aim, electronic structure calculations using density functional theory (DFT) was used to evaluate fault energies through direct simulations of faults and though indirect means involving analytical models. In the first part of the thesis, the effect of composition on the planar fault energies, elastic anisotropy, deformation modes and yield strength anomaly were explored in five pseudo-binaries (Ni, Co)3(Al,X) compositions of γ' (L12), where X = Ti, Ta, W, Ni. It was observed fault energies and deformation modes are sensitive to composition. The results are in good agreement with literature and can provide explanations for the deformation behaviour of both (Ni)3(Al, X) and Co3(Al,W). Though the results are encouraging, the above DFT calculations of fault energies are time-consuming, highlighting the need for high throughput models. In the second part of the thesis, such a model was developed to estimate these energies in well-known and novel A3B compositions. The new model treats the planar fault as a diffuse interface and allows estimation of fault energy in terms of energy of geometrically close packed structures with the same A3B composition and a bonding environment akin to that of the fault. The proposed model was used to predict energies of different superlattice faults in over 40 A3B compounds. The model was found to be highly accurate even without use of fitting parameters and has a fifteen-fold computational advantage over direct simulation. The model was extended to novel A3B compounds based on Pt3X, Rh3X and Ir3X where data is presently lacking. Despite the efficiency of the model, it had limitations in predicting fault energies in non-binary compositions. To account for this, in the last section of this thesis, a novel quasi-chemical model incorporating the far-field composition effects, was developed. The model was validated for several pseudo-binary systems and it was found that be more accurate than the diffuse interface model. The two models were then extended to predict fault energies in binary L12-ordered A3B compounds at high temperatures, complex multicomponent L12-ordered A3B compounds, and binary D019-ordered A3B compounds.